# Relative Humidity Calculator

Calculate relative humidity from air temperature and dew point. See absolute humidity, comfort level, and fog risk based on temperature-dew point spread.

## What this calculates

Relative humidity tells you how close the air is to being fully saturated with water vapor. At 85°F with a dew point of 65°F, the relative humidity is about 51%. This calculator uses the Magnus formula to compute RH from air temperature and dew point, and it also shows absolute humidity and a comfort assessment.

## Inputs

- **Air Temperature** (°F) — min -60, max 140 — Current air temperature.
- **Dew Point Temperature** (°F) — min -60, max 140 — Dew point temperature (must be at or below air temperature).

## Outputs

- **Relative Humidity** (%) — Relative humidity as a percentage.
- **Absolute Humidity** (g/m³) — Mass of water vapor per cubic meter of air.
- **Comfort Level** — formatted as text — Subjective comfort based on dew point.
- **Temp-Dew Point Spread** (°C) — Difference between air temp and dew point (fog risk indicator).

## Details

**The formula:**

RH = (es(Td) / es(T)) × 100

Where es is the saturation vapor pressure, T is the air temperature, and Td is the dew point. The saturation vapor pressure is calculated using the Magnus formula: es = 6.1078 × exp((17.27 × T) / (237.7 + T)).

**What relative humidity actually means:**

At 50% RH, the air holds half the water vapor it could at that temperature. Warming the air increases its capacity, so the same amount of moisture in the morning (when it is cool and RH is high) gives a lower RH in the afternoon (when it is warm).

**Relative vs. absolute humidity:**

Relative humidity depends on temperature, which makes it confusing. Air at 90°F and 50% RH contains far more moisture than air at 40°F and 50% RH. Absolute humidity (grams of water per cubic meter) is a direct measure that does not change with temperature.

**Fog prediction:**

When the temperature-dew point spread narrows to 2-3°C (4-5°F), fog is likely. Pilots, drivers, and weather forecasters watch this spread closely during evening and early morning hours.

**Comfortable humidity ranges:**
- **Indoor comfort:** 30-50% RH is ideal
- **Below 30%:** Dry skin, static electricity, respiratory irritation
- **Above 60%:** Mold growth risk, musty conditions
- **Above 70%:** Condensation on windows, potential structural damage

## Frequently Asked Questions

**Q: How do I calculate relative humidity from temperature and dew point?**

A: Calculate the saturation vapor pressure at both the air temperature and the dew point using the Magnus formula. Then divide the dew point vapor pressure by the air temperature vapor pressure and multiply by 100. This gives relative humidity as a percentage.

**Q: What is the difference between relative and absolute humidity?**

A: Relative humidity is a percentage that depends on temperature. The same air at 50% RH and 90°F holds much more water than air at 50% RH and 50°F. Absolute humidity (grams per cubic meter) measures the actual mass of water vapor and does not change with temperature alone.

**Q: Can relative humidity be over 100%?**

A: In theory, no. In practice, supersaturation can briefly occur (up to about 101-102%) before condensation catches up. For all practical purposes, 100% is the maximum, and it means the air is fully saturated at that temperature.

**Q: Why is relative humidity higher in the morning?**

A: Because the air is cooler in the morning. The dew point (absolute moisture) stays roughly constant overnight, but as the temperature drops toward the dew point, the relative humidity rises. By morning, RH is often 90-100%, which is why dew and fog are common at dawn.

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Source: https://vastcalc.com/calculators/physics/relative-humidity
Category: Physics
Last updated: 2026-04-08